Assertion: Total number is an easily adoptable measure if the populations is huge and counting is impossible or very time consuming.
Reason: In a dense laboratory culture of bacteria in a petri dish the best measures to report its density is number or biomass.

If you have a dense laboratory culture of bacteria in a petri what is the best measure to report its density ?

Assertion: The number of fish caught per trap is good enough measure of its populations density in the lake. Reason: For certain ecological investigations, there is no need to know the absolute populations densities, relative densities serve the purpose equally well.